Fuel cells (FCs) are systems which convert the heat energy of the fuel, generally hydrogen or an alcohol, to electrical energy via two electrochemical reactions.
One of these reactions is the reaction for oxidation of the fuel (H2 or alcohols) at the anode, which generates electrons by the following reaction:H2→2(H++e−).
The other reaction is the reaction for reduction of the oxidant, pure O2 or air, which takes place at the cathode and which generates water by the following reaction:½O2+2(H++e−)→H2O.
This reaction is a catalyzed reaction, generally catalyzed by platinum which is deposited on the electrode, forming what is referred to as an active layer.
Thus, fuel cells can only operate if the ionic and electrical charges are transported as far as the cathode.
The transfer of the proton charges formed at the anode is provided in general by a membrane composed of a proton-exchange polymer, for example made of Nafion®, which is a copolymer based on sulfonated tetrafluoroethylene polymer. In the case of the Nafion®, this transfer is provided by virtue of its sulfonated chains.
The conduction of the electrons takes place for its part via electric wires and makes it possible to convey the electrons as far as the cathode.
The work carried out by the displacement of these electrons provides the energy necessary for the operation, for example, of an electrical appliance.
Nevertheless, the low kinetics of the oxygen reduction reaction (ORR), which takes place at the cathode, results in the electrodes being charged with a high content of catalyst, which is generally platinum. This platinum content is of the order of 0.6 to 0.8 mg/cm2.
These high charges significantly increase the cost of the fuel cells and make these cells less competitive in the face of conventional thermal systems, in particular for motor vehicle applications.
Furthermore, the conventional methods for the preparation of the electrodes have the major disadvantage of having a low degree of use of the platinum. This means that the random distribution of the platinum particles in the active layer does not make it possible to optimize the reaction surface for the oxygen reduction reaction taking place at the cathode.
This means, a contrario, that a “nano-organized” distribution should make it possible to reduce the charging with platinum to approximately from 0.15 to 0.2 mg/cm2 while retaining a high catalytic performance, for example for a motor vehicle application.
As indicated in the cathodic equation of the ORR, the degree of use of the platinum is at a maximum if the combined catalytic sites correspond to the encounter of a triple contact between the gas, the electrons and the protons.
The commonest method for the preparation of a fuel cell electrode consists in impregnating a carbon powder with a solution of metal salts and in then reducing the metal ions with reducing agents, such as sodium borohydride, formaldehyde, ethylene glycol, and the like, this carbon-based solution subsequently being filtered and subsequently being subjected to a heat treatment.
The powder obtained comprises metal nanoparticles (generally platinum nanoparticles) with sizes of between 2 and 5 nm. This catalytic powder thus obtained is mixed with water, isopropanol and a solution of the proton-exchange polymer, for example made of Nafion®, and becomes an ink which is applied to a diffusion layer to become a gas diffusion electrode. These prepared electrodes (one for the anode and one for the cathode) are assembled on either side of a membrane composed of a proton-exchange polymer solution, for example made of Nafion®, and hot pressed in order to obtain a membrane electrode assembly (MEA).
These electrodes have active layers with a thickness of 50 μm, whereas only the first micrometers close to the membrane composed of a proton-exchange polymer, for example made of Nafion®, actively participate in the electrochemical reactions.
In order to overcome this problem, provision has been made to use an electrochemical deposition method which makes it possible to concentrate the active layer in the first micrometers of the diffusion layer and to avoid dispersing catalytic particles in the body of the electrode rather than at the surface.
Provision has also been made to use deposition techniques, such as physical vapor deposition or chemical vapor deposition or metalorganic vapor phase deposition, to form the active layer.
Although these techniques have been shown useful for small systems, they cannot be readily envisaged for larger systems due to the cumbersomeness of the experimental means to be used and the cost related to these processes.
Thus, the electrochemical deposition technique is an economically viable and simple to employ process which makes it possible to reduce the charging with platinum of FC electrodes.
However, in terms of localization, this technique, although it makes it possible to make sure that the catalyst is found in regions accessible to the electrons, does not make it possible to make sure that the catalyst is accessible to the gas and to the protons.